The FBI is investigating allegations that Sen. Rob. Menendez, D-N.J., patronized prostitutes in the Dominican Republic, two people familiar with the investigation told the Washington Post.

One source told the Post that investigators are focused on whether Salomon Melgen, a Florida eye doctor and major Menendez donor, provided the Menendez with prostitutes while he was vacationing in the Caribbean nation. Another person indicated that the FBI is looking into allegations of underage prostitution and sex parties.

The FBI, according to the sources, was tipped off by a series of anonymous emails alleging that Menendez had patronized the prostitutes while vacationing at Melgen's resort home in the Dominican Republic. Prostitution is not illegal in the Dominican Republic, but solicitation of underage prostitutes is.

According to the Post, the FBI is also investigating whether Menendez has improperly pressured U.S. officials to lobby for a port security contract in the island nation that would benefit Melgen, who is invested in the company holding the contract. A politician who accepts gifts or favors in exchange for official acts can be prosecuted for abusing public office.

Menendez has fiercely denied the allegations, saying, "All of those smears are absolutely false, and that's the bottom line."

The prostitution claims have been swirling since last Spring, when the anonymous tipster alerted a government watchdog group, Citizens for Responsibility and Ethics in Washington (CREW), of the allegations. Despite some questions about the veracity of the tipster's claims - the organization could not get the individual to meet or speak on the telephone - CREW forwarded the allegations to the FBI in July.

The tipster's letters included the names of several women who had allegedly had improper relations with Menendez and Melgen, but several of the women named have come forward to deny any connection to the matter. One woman, Svitlana Buchyk, told the Miami Herald that she worked for Melgen and considered him a "generous person," but her lawyer, Gerald Greenberg, explained to the Washington Post, "Ms. Buchyk is an actress and model attending to her career. Any allegation that she had an improper relationship with Senator Menendez or anyone else is sheer nonsense."

An FBI spokesman and Melgen and his attorneys declined to comment for the Post article.

Menendez's relationship with his donor and friend Melgen has been the basis of several recent snafus. The senator admitted this month that he did not properly disclose two 2010 trips on Melgen's private plane to Melgen's Dominican villa. The Senate Ethics Committee is looking into the matter.

Melgen is also the subject of a separate investigation examining whether he defrauded the government by overcharging Medicare for millions of dollars in unnecessary treatments. The FBI raided Melgen's Palm Beach clinic on Jan. 29, collecting boxes of documents pursuant to their inquiry.